Abstract:

Odontocetes are assumed to use echolocation for navigation and foraging, but
neither of these uses of biosonar has been conclusively demonstrated in free-ranging
animals. Many bats are known to use echolocation throughout foraging sequences,
changing the structure and timing of clicks as they progress towards prey capture. For
odontocetes, however, we do not know enough about their foraging behavior to describe
such sequences. To conduct detailed behavioral observations of any subject animal, the
observer must be able to maintain continuous visual contact with the subject for a period
commensurate with the duration of the behavior(s) of interest. Behavioral studies of
cetaceans, which spend approximately 95% of their time below the water's surface, have
been limited to sampling surface behavior except in special circumstances, e.g. clear-water
environments, or with the use of technological tools. I addressed this limitation
through development of an observation platform consisting of a remote controlled video
camera suspended from a tethered airship with boat-based monitoring, adjustment, and
recording of video. The system was used successfully to conduct continuous behavioral
observations of bottlenose dolphins in the Sarasota Bay, FL area. This system allowed
me to describe previously unreported foraging behaviors and elucidate functions for
behaviors already defined but poorly understood. Dolphin foraging was modeled as a
stage-structured sequence of behaviors, with the goal-directed feeding event occurring at
the end of a series of search, encounter, and pursuit behaviors. The behaviors preceding a
feeding event do not occur in a deterministic sequence, but are adaptive and plastic. A
single-step transition analysis beginning with prey capture and receding in time has
identified significant links between observed behaviors and demonstrated the stage-structured
nature of dolphin foraging. Factors affecting the occurrence of specific
behaviors and behavioral transitions include mesoscale habitat variation and individual
preferences. The role of sound in foraging, especially echolocation, is less well understood
than the behavioral component. Recent studies have explored the use of echolocation in
captive odontocete foraging and presumed feeding in wild animals, but simultaneous,
detailed behavioral and acoustic observations have eluded researchers. The current study used two methods to obtain acoustic data. The overhead video system includes two
towed hydrophones used to record 'ambient' sounds of dolphin foraging. The recordings
are of the 'ambient' sounds because the source of the sounds, i.e. animal, could not be
localized. Many focal follows, however, were conducted with single animals, and from
these records the timing of echolocation and other sounds relative to the foraging
sequence could be examined. The 'ambient' recordings revealed that single animals are
much more vocal than animals in groups, both overall and during foraging. When not
foraging, single animals vocalized at a rate similar to the per animal rate in groups of ≥2
animals. For single foraging animals, the use of different sound types varies significantly
by the habitat in which the animal is foraging. These patterns of use coupled with the
characteristics of the different sound types suggest specific functions for each. The
presence of multiple animals in a foraging group apparently reduces the need to vocalize,
and potential reasons for this pattern are discussed. In addition, the increased vocal
activity of single foraging animals lends support to specific hypotheses of sound use in
bottlenose dolphins and odontocetes in general. The second acoustic data collection
method records sounds known to be from a specific animal. An acoustic recording tag
was developed that records all sounds produced by an animal including every
echolocation click. The tag also includes an acoustic sampling interval controller and a
sensor suite that measures pitch, roll, heading, and surfacing events. While no foraging
events occurred while an animal was wearing an acoustic data logger, the rates of
echolocation and whistling during different activities, e.g. traveling, were measured.

Description:

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999